1. Field of the Invention
This invention relates to a fastening system for assembling plastic units and more particularly to spring steel clips for retaining attaching hardware in the assembly of molded plastic structures. The clips are utilizable with corresponding plastic receptors, such as ribs, which ribs, when employed in lieu of tubular plastic posts or bosses, avoid the occurrence of sink marks and sink holes opposite the attachment site. The clips of this invention also support repetitive assembly and disassembly.
2. Description of Prior Art
The connecting of large- or medium-scale molded plastic parts to one another has been fraught with technical problems for many years. In the 1940's, Tinnerman developed a fastener and a fastener company, Tinnerman Products, Inc., which aided assembly of such parts by the introduction of the Tinnerman nut. Tinnerman's success is memorialized by several patents including U.S. Pat. Nos. 2,326,579 and 2,344,570.
In many applications the boss-caused sink marks and sink holes are irrelevant as the sink holes do not interfere with the functionality or esthetics of the product and as the products are not routinely cleaned or sterilized. However, with products requiring highly esthetic finishes and fine cosmetic appearance, smooth surfaces without valleys or dimples are significant. Of course, in medical and personal hygiene products such surfaces are important as cleaning and sterilization are facilitated thereby.
In the design of modern-day plastic assemblies, attention is drawn to the overall recyclable aspects thereof. Where assemblies with molded-in-place brass inserts are used, recycling becomes difficult because of the salvage labor required to remove the inserts. Thus, in mis-manufactured plastic parts where an insert has been missed or insufficient resin is supplied to the mold or a color defect exists, regranulation becomes uneconomic. The use of metal inserts, while permitting repetitive assembly and disassembly, still remains problematic with regard to recycling and regranulation of waste plastic.
Another fastener arrangement, as shown in Levey et al, U.S. Pat. No. 6,976,818, assigned to Illinois Tool Works Inc., is the plastic boss and the corresponding screw. Here, without using an insert, an interlocking screw is driven into the plastic and cuts a swath in the plastic body of the boss. Various thread designs are known in the art and are configured for cutting and interlocking with the boss. Each time the plastic assembly is assembled and disassembled, additional cutting takes place. Unlike the metal insert discussed above, the plastic boss and screw fastener arrangement does not support repetitive assembly and disassembly.
The designing of bosses for mounting purposes is an inexact science that has given rise to numerous rules of thumb for avoiding sink marks and holes. Such rules provide: (1) the boss height should not be more than twice the diameter; (2) a radius should be provided where the boss joins the wall; and, (3) the boss diameter should be smaller in dimension than the thickness of the wall. While all the rules are helpful in enabling the mold designer to practice his art, the underlying problem is maintaining a suitable low mass of plastic for mounting purposes on one side of a plastic wall to avoid sink marks and holes on the other side.
As the automotive industry developed, many spring clips were designed to enhance assembly line operations. Among the earliest were those of Fred S. Carr and United-Carr Fastener Corporation which corporation worked closely with Fisher Body. Typical of the devices is the invention of F. E. Johnson, U.S. Pat. No. 2,295,480 providing a nut mounted on a clip for assisting panel mounting. A Ford Motor Company development provided a sliding clip of W. D. Gaw Jr, U.S. Pat. No. 5,536,125 to enable axial alignment. More recently, in a patent to L. P. Tanner, U.S. Pat. No. 6,796,760 a retention clip is disclosed. This U-shaped clip has a holding arm restraining lateral movement of a captive fastener during shipment of a panel (with fasteners held thereon).
In the course of preparing for this application, the inventor hereof became aware of the following patents:
U.S.Pat. No.InventorIssue Date6,976,818Levey et al.Dec. 20, 20056,899,499Dickinson et al.May 31, 20056,848,874Lowry et al.Feb. 1, 20056,796,760TannerSep. 28, 20046,283,689Roytberg et al.Sep. 4, 20015,855,461TripiJan. 5, 19995,536,125Gaw, Jr.Jul. 16, 19963,400,743StrangeSep. 10, 19682,344,570TinnermanMar. 21, 19442,326,579TinnermanAug. 10, 19432,295,480JohnsonSep. 8, 1942The patents not mentioned hereinabove are considered briefly below.
Dickinson et al—U.S. Pat. No. 6,899,499—issued May 31, 2005 Dickinson '499 discloses a fastener that snaps into a slot-containing structure attached to the host portion of the unit being attached. The structure required to snap the fastener into place necessarily has a high mass, running a high risk for sink marks and holes. The metal fastener thereof is not readily removable for the recycling of the plastic.
Lowry et al—U.S. Pat. No. 6,848,874—issued Feb. 1, 2005 Lowry '874 discloses a spring fastener with a configuration of barbs that allows for the extraction of the fastener without the destruction of the slot or the fastener. However, since the fastener must be installed in a slot, the construction that would be required to use such a fastener in the present application, without puncturing the host portion of the plastic unit, would necessarily have a high mass, thereby causing the formation of sink marks and holes on the opposite side of the wall.
Tanner—U.S. Pat. No. 6,796,760—issued Sep. 28, 2004 Tanner '760 discloses a retainer for a captive fastener during transportation thereof. It is suitable for application to automotive technology, particularly for panels with pre-inserted fasteners, which do not require repetitive insertion and removal.
Roytberg et al—U.S. Pat. No. 6,283,689—issued Sep. 4, 2001 Roytberg '689 discloses a one-piece insert fastener with outwardly protruding wings, which wings are secured within an aperture in the outer surface of a panel.
Tripi—U.S. Pat. No. 5,855,461—issued Jan. 5, 1999 Tripi '461 discloses an anchor for securing a threaded fastener, which provides for repetitive assembly and disassembly, and is suitable for application to automotive technology, particularly for panels. The anchor of this invention is for mounting in a circular hole.
Gaw, Jr.—U.S. Pat. No. 5,536,125—issued Jul. 16, 1996 Gaw Jr. '125 discloses a fastening assembly for securing plastic panels to metallic substrates. As part of the fastening assembly, a nut is snap-fit mounted to a plastic insert, which plastic insert in turn is snapped into the substrate. The fastening assembly is suitable for application to automotive assembly and provides for repetitive assembly and disassembly.
Strange—U.S. Pat. No. 3,400,743—issued Sep. 10, 1968 Strange '743 discloses a fastener that includes a screw and a sheetmetal nut holder with resilient arms. The retention force of the resilient arms results from the screw insertion and spreading of the arms thereby. Thus, upon the removal of the screw, the nut is freed.
Johnson—U.S. Pat. No. 2,295,480—issued Sep. 8, 1942 Johnson '480 discloses a nut device that requires a hole for installation of the nut. Thus, for use in the present application, where a puncture is to be avoided, the molding of a boss is required.
In viewing the prior art, several fastener system and fastener characteristics have been considered, namely: (1) the likelihood of the formation of sink marks and sink holes on the plastic unit being assembled; (2) the effect of repetitive disassembly and reassembly on the structure; and, (3) the ease of recapturing and regranulating plastic from mismanufactured units. None of the above configurations satisfy all of these desired criteria.
It will be seen that the rib clip of this invention both considered alone and together with the other components of the fastener system provide an answer to the long felt need expressed by the above criteria.